PROJECTSUMMARY Persistent muscle weakness and physical disability decrease quality of life for millions of Americans. Thesephysicallimitationsarelinkedwithpathologicalaccumulationofinter/intramuscularadiposetissue(IMAT) across numerouspopulations and morbidities. This link combined withthe resistanceofIMAT accumulationto standard diet and exercise remodeling cues points to IMAT as being a persistent and unaddressed clinical problem, likely impeding rehabilitativeefforts to maintain or increase muscle strength. The long-term objective of this research is to leverage our understanding of fat-muscle cross-talk to therapeutically modulate IMAT paracrine signaling to promote muscle growth and regeneration. Recently, IMAT has been identified as a specialized sub-type of fat called beige fat, that can be stimulated to adopt features of brown fat, potentially including the secretion of pro-regenerative factors. It is the primary objective of this proposal to determine the mechanismsofpro-regenerativeactionofbrownfatonmuscleandwhetherthisactioncanbemimickedbyIMAT stimulation. Our central hypothesis is that brown fat promotes muscle regeneration by enhancing myogenic differentiation via paracrine secreted follistatin. To address our hypothesis, our approach will use our novel systemofmouseintermuscularfattransplanttomodelparacrinefat-musclecross-talk.Wewillusethissystem tocontrolthetimingandcompositionofbrownfatsignalingformechanisticexplorationofthecellularresponse during muscle regeneration (aim 1) and the role of follistatin in mediating that response (aim 2). We will also explorethetherapeuticpotentialofmimickingbrownfatparacrinesignalingviastimulationinmouseandhuman IMAT(aim3).Altogether,thesestudieswillbroadenourunderstandingofthebasicbiologyofbrownfatsignaling, thenatureofIMAT-musclecross-talkandthetherapeuticpotentialforstimulatingIMATandotherdepotsofbeige fattoimprovetissuephysiology.